Cross-Talk of Toll-Like Receptor 5 and Mu-Opioid Receptor Attenuates Chronic Constriction Injury-Induced Mechanical Hyperalgesia through a Protein Kinase C Alpha-Dependent Signaling.

Ching Chang,Ming-Lin Yang,Chao-Bin Yeh,To-Jung Tseng,Chiung-Hui Liu,Hung-Kai Liu,Wen-Chieh Liao

doi:10.3390/ijms22041891

Abstract

Recently, Toll-like receptors (TLRs), a family of pattern recognition receptors, are reported as potential modulators for neuropathic pain; however, the desired mechanism is still unexplained. Here, we operated on the sciatic nerve to establish a pre-clinical rodent model of chronic constriction injury (CCI) in Sprague-Dawley rats, which were assigned into CCI and Decompression groups randomly. In Decompression group, the rats were performed with nerve decompression at post-operative week 4. Mechanical hyperalgesia and mechanical allodynia were obviously attenuated after a month. Toll-like receptor 5 (TLR5)-immunoreactive (ir) expression increased in dorsal horn, particularly in the inner part of lamina II. Additionally, substance P (SP) and isolectin B4 (IB4)-ir expressions, rather than calcitonin-gene-related peptide (CGRP)-ir expression, increased in their distinct laminae. Double immunofluorescence proved that increased TLR5-ir expression was co-expressed mainly with IB4-ir expression. Through an intrathecal administration with FLA-ST Ultrapure (a TLR5 agonist, purified flagellin from Salmonella Typhimurium, only the CCI-induced mechanical hyperalgesia was attenuated dose-dependently. Moreover, we confirmed that mu-opioid receptor (MOR) and phospho-protein kinase Cα (pPKCα)-ir expressions but not phospho-protein kinase A RII (pPKA RII)-ir expression, increased in lamina II, where they mostly co-expressed with IB4-ir expression. Go 6976, a potent protein kinase C inhibitor, effectively reversed the FLA-ST Ultrapure- or DAMGO-mediated attenuated trend towards mechanical hyperalgesia by an intrathecal administration in CCI rats. In summary, our current findings suggest that nerve decompression improves CCI-induced mechanical hyperalgesia that might be through the cross-talk of TLR5 and MOR in a PKCα-dependent manner, which opens a novel opportunity for the development of analgesic therapeutics in neuropathic pain.

Highlights

Neuropathic pain is a neurological disorder caused by dramatic physical injury to peripheral or central primary afferents which disturbs life quality extensively [1,2]
In constriction injury (CCI) group, the decreases of withdrawal threshold were revealed on ipsilateral sides, indicating mechanical hyperalgesia, from post-operative week (POW) 2 to POW 8 (F(3,46) = 259.9; p < 0.0001) (Figure 1A)
To explore whether PKCα contributes to TLR5- or MOR-mediated attenuation of pain hypersensitivity in a chronic pain state, we evaluated the CCI rats after an intrathecal administration with FLA-ST Ultrapure or DAMGO and in combined with Go 6976 at POW 8 (Figure 13)

Summary

Introduction

Neuropathic pain is a neurological disorder caused by dramatic physical injury to peripheral or central primary afferents which disturbs life quality extensively [1,2]. Central sensitization is defined as the pre-synaptic terminals increase in excitability or decrease in inhibition that modulate the post-synaptic neurons after peripheral nerve injury [5,6]. The pre-synaptic terminals in the distinct laminae of dorsal horn are critical to establish the synaptic plasticity of the spinal cord [6,7]. The pre-clinical rodent models such as chronic constriction injury (CCI), spared nerve injury (SNI) and lumbar 5 spinal nerve ligation (SNL) are established for investigating potential mechanisms in central sensitization [11,12,13]

Methods

Results

Conclusion